1. Field of the Invention
The present invention relates to a droplet determination device and a droplet determination method for a droplet discharge apparatus, and more particularly, to a droplet determination device and droplet determination method for determining the discharge of droplets in a droplet discharge apparatus which performs image recording by discharging droplets of ink, or the like, onto a recording medium.
2. Description of the Related Art
Conventionally, an image forming apparatus (inkjet printer) is known, which comprises an inkjet head (ink discharge head) having an arrangement of a plurality of nozzles (ink discharge ports) and which forms images on a recording medium by discharging ink (ink droplets) from the nozzles while causing the inkjet head and the recording medium to move relatively to each other.
Various methods are known conventionally as ink discharge methods for an inkjet recording apparatus of this kind. For example, one known method is a piezoelectric method, where the volume of a pressure chamber (ink chamber) is changed by causing a vibration plate forming a portion of the pressure chamber to deform due to deformation of a piezoelectric element (piezoelectric actuator), ink being introduced into the pressure chamber from an ink supply passage when the volume is increased, and the ink inside the pressure chamber being discharged as a droplet from the nozzle when the volume of the pressure chamber is reduced. Another known method is a thermal inkjet method where ink is heated to generate a bubble in the ink, and ink is then discharged by means of the expansive energy created as the bubble grows.
In an image forming apparatus having an ink discharge head such as an inkjet recording apparatus, ink is supplied to an ink discharge head via an ink supply channel from an ink tank which stores ink, and this ink is discharged by one of the various discharge methods described above. However, it is necessary that ink is discharged stably in such a manner that factors such as the ink discharge volume, the discharge velocity, the discharge direction, and the three-dimensional shape of the discharged ink, conform to prescribed values at all times.
However, during printing, ink is filled into the nozzles of the ink discharge head at all times, in order that printing can be carried out immediately upon receiving a print instruction. Since the ink inside the nozzles is exposed to the air, ink inside the nozzles which have not discharged ink for a long period of time proceeds to dry, the viscosity of the ink increases, and the nozzles may become blocked up. Furthermore, if air bubbles become trapped inside the ink supply channels, or the like, of if the ink supply is interrupted or discharge continues for a long period of time, then ink refilling slows down, the ink meniscus at the nozzle section retreats, and eventually, air bubbles may be sucked into the head due to the negative pressure on the ink supply side.
Due to reasons such as these, it is necessary to perform maintenance of the discharge head when ink is no longer being discharged in a stable fashion as described above. Therefore, conventionally, various methods have been proposed in order to determine whether or not ink is being discharged stably.
For example, a method is known in which a light source including an infrared LED array having a peak wavelength in the infrared region is disposed at one end of the front face of the discharge port surface (the nozzle surface), an infrared CCD sensor having a peak wavelength in the infrared region is disposed at the other end thereof, and a dummy discharge is performed toward an ink receptacle after each certain number of scans, the transmitted light being recorded by the light source and sensor, and the positions of discharge failures being determined on the basis of the density data thus recorded (see, for example, Japanese Patent Application Publication No. 06-270414).
Furthermore, a method is known in which the emission of small droplets and the direction of these droplets is checked one at a time for each of the nozzles of a nozzle array in a full-width array print head, by means of a droplet sensor including an infrared light-emitting diode (LED) and a sensing region formed by a single laterally placed photodiode (see, for example, Japanese Patent Application Publication No. 08-118679).
Moreover, a method is known in which the output of a sensor that detects the presence of ink droplets emitted from an inkjet head is integrated by an integrator, the integrated signal is amplified by a high-gain amplifier, thereby generating a sensor circuit output signal, and the emission of ink droplets is determined on the basis of this integrated output signal, which indicates the presence of a droplet when a droplet at least partially interrupts the light path (see, for example, U.S. Pat. No. 5,304,814).
Furthermore, a device is also known in which an infrared LED is used as a light-emitting element of a photosensor, a lens being formed integrally with the light emitting surface of the LED in such a manner that substantially parallel light is transmitted, and a phototransistor is used as a light-receiving element of the photosensor, the light-receiving surface of the light-receiving element being formed with a hole of 0.7 mm width and height by means of a molded member, thereby restricting the detection range, reducing the number of droplets that are detected simultaneously and hence increasing the detection resolution (see, for example, Japanese Patent Application Publication No. 08-309964). A device is known which has a similar composition, wherein the detection range is restricted to 2 mm in the height direction and 0.5 mm in the width direction, through the whole region between the light-receiving element and the light-emitting element, in such a manner that the detection width can be changed in the vertical and horizontal directions (see, for example, Japanese Patent Application Publication No. 09-94947).
Furthermore, a method is also known in which a light-emitting element and a light-receiving element of a photosensor for detecting ink discharge are positioned in such a manner that the optical axis of detection determined by these elements forms a prescribed angle with respect to the direction in which ink discharge ports are arranged in a recording head. Consequently, the detection region of the photosensor is increased (see, for example, Japanese Patent Application Publication No. 09-94948).
Moreover, a method is also known in which a light-emitting element and a light-receiving element for detecting ink discharged by an inkjet head are provided at a prescribed position within the range of movement of the inkjet head. If the amount of light arriving at the light-receiving element is reduced by the presence of an ink droplet discharge by the inkjet head, then a change in the current corresponding to this light reduction is amplified by a change amplifying unit, and the ink discharge amount is judged on the basis of a pulse signal which compares the amplified signal with a prescribed voltage (see, for example, Japanese Patent Application Publication No. 09-94959).
Furthermore, a device for inspecting missing dots by investigating whether or not an ink droplet passes between a light-emitting element and a light-receiving element is also known, in which the light emitting element is a laser that emits a light beam having an external diameter of approximate 1 mm or less, and the emitted laser light is shaped into a laser light beam of a prescribed thickness by cutting out the low-intensity fringe portions of the beam by means of a slit, in order to facilitate measurement of the flight velocity of the ink droplets, (namely, the light is shaped into a flat parallel light beam in which the top and bottom sides are cut off in a straight line). By measuring the time taken by the ink droplet to travel through the thickness of the laser light beam, it is possible to identify the flight velocity of the ink droplets (see, for example, Japanese Patent Application Publication No. 2000-272134).
A method is also known in which a light beam emitted by a light emitting element is directed toward a light-receiving element via a slit opening, and the slit is rotated to an angle corresponding to the size of a discharge ink droplet by automatically rotating the slit with respect to the light beam. By effectively changing the size of the slit opening in this way (in other words, changing the aperture width), an optimum light reception intensity is achieved in accordance with the size of the ink droplet, and therefore the state of ink discharge can be determined with good accuracy (see, for example, Japanese Patent Application Publication No. 2001-113681).
A method is also known in which a device for determining discharge characteristics data relating to ink droplets determines the presence or absence of discharge by detecting whether or not an ink droplet has interrupted light, and determines the volume of the ink droplet by measuring the amount of light interrupted by the ink droplet. Two parallel light beams are output from a light source, and the discharge velocity of the ink droplet is determined by measuring the time taken for the ink droplet to travel between these two light beams (see, for example, Japanese Patent Application Publication No. 2003-127430).
However, in the device described in Japanese Patent Application Publication No. 06-270414, the sensing device for determining the discharge state of the discharge ports, in other words, the sensor which records the transmitted light image does not comprise a single element, but rather, a plurality of pixels arranged in a line, as in a CCD. This is not sufficient in terms of improving determination sensitivity. Furthermore, the method described in Japanese Patent Application Publication No. 08-118679 actually detects droplets by means of a sensor formed by a single LED and photodiode, but it checks the nozzles one at a time and is not satisfactory in terms of achieving efficient inspection of a plurality of nozzles, or improving inspection sensitivity.
Furthermore, the methods and devices described in U.S. Pat. No. 5,304,814, Japanese Patent Application Publication No. 08-309964 and Japanese Patent Application Publication No. 09-94947 seek to improve determination sensitivity by means of a signal processing method, or by restricting the beam used for detection so as to reduce the number of ink droplets detected simultaneously, but they are not satisfactory in terms of improving determination sensitivity with respect to a head having a very large number of nozzles, such as a page-wide head which corresponds the full width of the recording medium, for example.
The method described in Japanese Patent Application Publication No. 09-94948 is a method for inspecting an entire inkjet head in a shuttle scanning system, by means of a fixed determination system, but this method is not effective for fixed, long heads, such as a single-pass page-wide head that is capable of recording onto the entire width of the recording medium in a single action, without shuttle scanning.
Furthermore, the method described in Japanese Patent Application Publication No. 09-94959 determines the ink discharge volume, but it is not able to determine the ink discharge velocity. If this method is combined with the method described in Japanese Patent Application Publication No. 2000-272134, then both the ink discharge volume and the ink discharge velocity can be determined, but since no compensatory method is provided for use in the case of differences in the ink droplet size, then it will not be possible to determine ink discharge velocity if the ink droplet size varies.
Furthermore, the method described in Japanese Patent Application Publication No. 2001-113681 changes the beam width, but the purpose of this is to stabilize the determination process by altering the light intensity, and it does not involve a significant change in determination characteristics. Furthermore, the method described in Japanese Patent Application Publication No. 2003-127430 requires two light beams in order to determine the ink discharge velocity, and hence the composition is complicated.